The ptrace() system call
provides tracing and debugging facilities. It allows one
process (the tracing process) to control another (the
traced process). The tracing process must first
attach to the traced process, and then issue a series of
ptrace() system calls to control the execution of the
process, as well as access process memory and register
state. For the duration of the tracing session, the traced
process will be ’’re-parented’’,
with its parent process ID (and resulting behavior) changed
to the tracing process. It is permissible for a tracing
process to attach to more than one other process at a time.
When the tracing process has completed its work, it must
detach the traced process; if a tracing process exits
without first detaching all processes it has attached, those
processes will be killed.

Most of the
time, the traced process runs normally, but when it receives
a signal (see sigaction(2)), it stops. The tracing process
is expected to notice this via wait(2) or the delivery of a
SIGCHLD signal, examine the state of the stopped process,
and cause it to terminate or continue as appropriate. The
signal may be a normal process signal, generated as a result
of traced process behavior, or use of the kill(2) system
call; alternatively, it may be generated by the tracing
facility as a result of attaching, stepping by the tracing
process, or an event in the traced process. The tracing
process may choose to intercept the signal, using it to
observe process behavior (such as SIGTRAP), or forward the
signal to the process if appropriate. The ptrace()
system call is the mechanism by which all this happens.

A traced process
may report additional signal stops corresponding to events
in the traced process. These additional signal stops are
reported as SIGTRAP or SIGSTOP signals. The tracing process
can use the PT_LWPINFO request to determine which events are
associated with a SIGTRAP or SIGSTOP signal. Note that
multiple events may be associated with a single signal. For
example, events indicated by the PL_FLAG_BORN,
PL_FLAG_FORKED, and PL_FLAG_EXEC flags are also reported as
a system call exit event (PL_FLAG_SCX). The signal stop for
a new child process enabled via PTRACE_FORK will report a
SIGSTOP signal. All other additional signal stops use
SIGTRAP.

Each traced
process has a tracing event mask. An event in the traced
process only reports a signal stop if the corresponding flag
is set in the tracing event mask. The current set of tracing
event flags include:

PTRACE_EXEC

Report a stop
for a successful invocation of execve(2). This event is
indicated by the PL_FLAG_EXEC flag in the pl_flags
member of struct ptrace_lwpinfo.

PTRACE_SCE

Report a stop
on each system call entry. This event is indicated by the
PL_FLAG_SCE flag in the pl_flags member of struct
ptrace_lwpinfo.

PTRACE_SCX

Report a stop
on each system call exit. This event is indicated by the
PL_FLAG_SCX flag in the pl_flags member of struct
ptrace_lwpinfo.

PTRACE_SYSCALL

Report stops
for both system call entry and exit.

PTRACE_FORK

This event flag
controls tracing for new child processes of a traced
process.

When this event
flag is enabled, new child processes will enable tracing and
stop before executing their first instruction. The new child
process will include the PL_FLAG_CHILD flag in the
pl_flags member of struct ptrace_lwpinfo. The
traced process will report a stop that includes the
PL_FLAG_FORKED flag. The process ID of the new child process
will also be present in the pl_child_pid member of
struct ptrace_lwpinfo. If the new child process was
created via vfork(2), the traced process’s stop will
also include the PL_FLAG_VFORKED flag. Note that new child
processes will be attached with the default tracing event
mask; they do not inherit the event mask of the traced
process.

When this event
flag is not enabled, new child processes will execute
without tracing enabled.

PTRACE_LWP

This event flag
controls tracing of LWP (kernel thread) creation and
destruction. When this event is enabled, new LWPs will stop
and report an event with PL_FLAG_BORN set before executing
their first instruction, and exiting LWPs will stop and
report an event with PL_FLAG_EXITED set before completing
their termination.

Note that new
processes do not report an event for the creation of their
initial thread, and exiting processes do not report an event
for the termination of the last thread.

When a thread
in the traced process creates a new child process via
vfork(2), the stop that reports PL_FLAG_FORKED and
PL_FLAG_SCX occurs just after the child process is created,
but before the thread waits for the child process to stop
sharing process memory. If a debugger is not tracing the new
child process, it must ensure that no breakpoints are
enabled in the shared process memory before detaching from
the new child process. This means that no breakpoints are
enabled in the parent process either.

The
PTRACE_VFORK flag enables a new stop that indicates when the
new child process stops sharing the process memory of the
parent process. A debugger can reinsert breakpoints in the
parent process and resume it in response to this event. This
event is indicated by setting the PL_FLAG_VFORK_DONE
flag.

The default
tracing event mask when attaching to a process via
PT_ATTACH, PT_TRACE_ME, or PTRACE_FORK includes only
PTRACE_EXEC events. All other event flags are disabled.

The
request argument specifies what operation is being
performed; the meaning of the rest of the arguments depends
on the operation, but except for one special case noted
below, all ptrace() calls are made by the tracing
process, and the pid argument specifies the process
ID of the traced process or a corresponding thread ID. The
request argument can be:

PT_TRACE_ME

This request is
the only one used by the traced process; it declares that
the process expects to be traced by its parent. All the
other arguments are ignored. (If the parent process does not
expect to trace the child, it will probably be rather
confused by the results; once the traced process stops, it
cannot be made to continue except via ptrace().) When
a process has used this request and calls execve(2) or any
of the routines built on it (such as execv(3)), it will stop
before executing the first instruction of the new image.
Also, any setuid or setgid bits on the executable being
executed will be ignored. If the child was created by
vfork(2) system call or rfork(2) call with the RFMEM flag
specified, the debugging events are reported to the parent
only after the execve(2) is executed.

PT_READ_I, PT_READ_D

These requests
read a single int of data from the traced
process’s address space. Traditionally,
ptrace() has allowed for machines with distinct
address spaces for instruction and data, which is why there
are two requests: conceptually, PT_READ_I reads from the
instruction space and PT_READ_D reads from the data space.
In the current FreeBSD implementation, these two requests
are completely identical. The addr argument specifies
the address (in the traced process’s virtual address
space) at which the read is to be done. This address does
not have to meet any alignment constraints. The value read
is returned as the return value from ptrace().

PT_WRITE_I, PT_WRITE_D

These requests parallel
PT_READ_I and PT_READ_D, except that they write rather than
read. The data argument supplies the value to be
written.

PT_IO

This request
allows reading and writing arbitrary amounts of data in the
traced process’s address space. The addr
argument specifies a pointer to a struct
ptrace_io_desc, which is defined as follows:

struct
ptrace_io_desc {

int

piod_op;

/* I/O operation */

void

*piod_offs;

/* child offset */

void

*piod_addr;

/* parent offset */

size_t

piod_len;

/* request length */

};

/*
* Operations in piod_op.
*/

#define PIOD_READ_D

1

/* Read from D space */

#define PIOD_WRITE_D

2

/* Write to D space */

#define PIOD_READ_I

3

/* Read from I space */

#define PIOD_WRITE_I

4

/* Write to I space */

The data
argument is ignored. The actual number of bytes read or
written is stored in piod_len upon return.

PT_CONTINUE

The traced
process continues execution. The addr argument is an
address specifying the place where execution is to be
resumed (a new value for the program counter), or (

caddr_t )1 to indicate that execution is to pick
up where it left off. The data argument provides a
signal number to be delivered to the traced process as it
resumes execution, or 0 if no signal is to be sent.

PT_STEP

The traced
process is single stepped one instruction. The addr
argument should be passed (

caddr_t )1. The data argument provides a
signal number to be delivered to the traced process as it
resumes execution, or 0 if no signal is to be sent.

PT_KILL

The traced
process terminates, as if PT_CONTINUE had been used with
SIGKILL given as the signal to be delivered.

PT_ATTACH

This request
allows a process to gain control of an otherwise unrelated
process and begin tracing it. It does not need any
cooperation from the to-be-traced process. In this case,
pid specifies the process ID of the to-be-traced
process, and the other two arguments are ignored. This
request requires that the target process must have the same
real UID as the tracing process, and that it must not be
executing a setuid or setgid executable. (If the tracing
process is running as root, these restrictions do not
apply.) The tracing process will see the newly-traced
process stop and may then control it as if it had been
traced all along.

PT_DETACH

This request is
like PT_CONTINUE, except that it does not allow specifying
an alternate place to continue execution, and after it
succeeds, the traced process is no longer traced and
continues execution normally.

PT_GETREGS

This request
reads the traced process’s machine registers into the
’’

struct reg ’’ (defined in
<machine/reg.h>) pointed to by addr.

PT_SETREGS

This request is
the converse of PT_GETREGS; it loads the traced
process’s machine registers from the
’’

struct reg ’’ (defined in
<machine/reg.h>) pointed to by addr.

PT_GETFPREGS

This request
reads the traced process’s floating-point registers
into the ’’

struct fpreg ’’ (defined in
<machine/reg.h>) pointed to by addr.

PT_SETFPREGS

This request is
the converse of PT_GETFPREGS; it loads the traced
process’s floating-point registers from the
’’

struct fpreg ’’ (defined in
<machine/reg.h>) pointed to by addr.

PT_GETDBREGS

This request
reads the traced process’s debug registers into the
’’

struct dbreg ’’ (defined in
<machine/reg.h>) pointed to by addr.

PT_SETDBREGS

This request is
the converse of PT_GETDBREGS; it loads the traced
process’s debug registers from the ’’

struct dbreg ’’ (defined in
<machine/reg.h>) pointed to by addr.

PT_LWPINFO

This request
can be used to obtain information about the kernel thread,
also known as light-weight process, that caused the traced
process to stop. The addr argument specifies a
pointer to a struct ptrace_lwpinfo, which is defined
as follows:

struct
ptrace_lwpinfo {

lwpid_t pl_lwpid;

int

pl_event;

int

pl_flags;

sigset_t pl_sigmask;

sigset_t pl_siglist;

siginfo_t pl_siginfo;

char

pl_tdname[MAXCOMLEN + 1];

pid_t

pl_child_pid;

u_int

pl_syscall_code;

u_int

pl_syscall_narg;

};

The data
argument is to be set to the size of the structure known to
the caller. This allows the structure to grow without
affecting older programs.

The thread stopped due to
system call entry, right after the kernel is entered. The
debugger may examine syscall arguments that are stored in
memory and registers according to the ABI of the current
process, and modify them, if needed.

PL_FLAG_SCX

The thread is stopped
immediately before syscall is returning to the usermode. The
debugger may examine system call return values in the
ABI-defined registers and/or memory.

PL_FLAG_EXEC

When PL_FLAG_SCX is set, this
flag may be additionally specified to inform that the
program being executed by debuggee process has been changed
by successful execution of a system call from the
execve(2) family.

PL_FLAG_SI

Indicates that
pl_siginfo member of struct ptrace_lwpinfo
contains valid information.

PL_FLAG_FORKED

Indicates that the process is
returning from a call to fork(2) that created
a new child process. The process identifier of the new
process is available in the pl_child_pid member of
struct ptrace_lwpinfo.

PL_FLAG_CHILD

The flag is set for first event
reported from a new child which is automatically attached
when PTRACE_FORK is enabled.

PL_FLAG_BORN

This flag is set for the first
event reported from a new LWP when PTRACE_LWP is enabled. It
is reported along with PL_FLAG_SCX.

PL_FLAG_EXITED

This flag is set for the last
event reported by an exiting LWP when PTRACE_LWP is enabled.
Note that this event is not reported when the last LWP in a
process exits. The termination of the last thread is
reported via a normal process exit event.

PL_FLAG_VFORKED

Indicates that the thread is
returning from a call to vfork(2) that created a new child
process. This flag is set in addition to PL_FLAG_FORKED.

PL_FLAG_VFORK_DONE

Indicates that the thread has
resumed after a child process created via vfork(2) has
stopped sharing its address space with the traced
process.

pl_sigmask

The current signal mask of the
LWP

pl_siglist

The current pending set of
signals for the LWP. Note that signals that are delivered to
the process would not appear on an LWP siglist until the
thread is selected for delivery.

pl_siginfo

The siginfo that accompanies
the signal pending. Only valid for PL_EVENT_SIGNAL stop when
PL_FLAG_SI is set in pl_flags.

pl_tdname

The name of the thread.

pl_child_pid

The process identifier of the
new child process. Only valid for a PL_EVENT_SIGNAL stop
when PL_FLAG_FORKED is set in pl_flags.

pl_syscall_code

The ABI-specific identifier of
the current system call. Note that for indirect system calls
this field reports the indirected system call. Only valid
when PL_FLAG_SCE or PL_FLAG_SCX is set in
pl_flags.

pl_syscall_narg

The number of arguments passed
to the current system call not counting the system call
identifier. Note that for indirect system calls this field
reports the arguments passed to the indirected system call.
Only valid when PL_FLAG_SCE or PL_FLAG_SCX is set in
pl_flags.

PT_GETNUMLWPS

This request
returns the number of kernel threads associated with the
traced process.

PT_GETLWPLIST

This request
can be used to get the current thread list. A pointer to an
array of type lwpid_t should be passed in
addr, with the array size specified by data.
The return value from ptrace() is the count of array
entries filled in.

PT_SETSTEP

This request
will turn on single stepping of the specified process.
Stepping is automatically disabled when a single step trap
is caught.

PT_CLEARSTEP

This request
will turn off single stepping of the specified process.

PT_SUSPEND

This request
will suspend the specified thread.

PT_RESUME

This request
will resume the specified thread.

PT_TO_SCE

This request
will set the PTRACE_SCE event flag to trace all future
system call entries and continue the process. The
addr and data arguments are used the same as
for PT_CONTINUE.

PT_TO_SCX

This request
will set the PTRACE_SCX event flag to trace all future
system call exits and continue the process. The addr
and data arguments are used the same as for
PT_CONTINUE.

PT_SYSCALL

This request
will set the PTRACE_SYSCALL event flag to trace all future
system call entries and exits and continue the process. The
addr and data arguments are used the same as
for PT_CONTINUE.

PT_GET_SC_ARGS

For the thread
which is stopped in either PL_FLAG_SCE or PL_FLAG_SCX state,
that is, on entry or exit to a syscall, this request fetches
the syscall arguments.

The arguments
are copied out into the buffer pointed to by the addr
pointer, sequentially. Each syscall argument is stored as
the machine word. Kernel copies out as many arguments as the
syscall accepts, see the pl_syscall_narg member of
the struct ptrace_lwpinfo, but not more than the
data bytes in total are copied.

PT_FOLLOW_FORK

This request
controls tracing for new child processes of a traced
process. If data is non-zero, PTRACE_FORK is set in
the traced process’s event tracing mask. If
data is zero, PTRACE_FORK is cleared from the traced
process’s event tracing mask.

PT_LWP_EVENTS

This request
controls tracing of LWP creation and destruction. If
data is non-zero, PTRACE_LWP is set in the traced
process’s event tracing mask. If data is zero,
PTRACE_LWP is cleared from the traced process’s event
tracing mask.

PT_GET_EVENT_MASK

This request
reads the traced process’s event tracing mask into the
integer pointed to by addr. The size of the integer
must be passed in data.

PT_SET_EVENT_MASK

This request
sets the traced process’s event tracing mask from the
integer pointed to by addr. The size of the integer
must be passed in data.

PT_VM_TIMESTAMP

This request
returns the generation number or timestamp of the memory map
of the traced process as the return value from
ptrace(). This provides a low-cost way for the
tracing process to determine if the VM map changed since the
last time this request was made.

PT_VM_ENTRY

This request is
used to iterate over the entries of the VM map of the traced
process. The addr argument specifies a pointer to a
struct ptrace_vm_entry, which is defined as
follows:

struct
ptrace_vm_entry {

int

pve_entry;

int

pve_timestamp;

u_long

pve_start;

u_long

pve_end;

u_long

pve_offset;

u_int

pve_prot;

u_int

pve_pathlen;

long

pve_fileid;

uint32_t

pve_fsid;

char

*pve_path;

};

The first entry
is returned by setting pve_entry to zero. Subsequent
entries are returned by leaving pve_entry unmodified
from the value returned by previous requests. The
pve_timestamp field can be used to detect changes to
the VM map while iterating over the entries. The tracing
process can then take appropriate action, such as
restarting. By setting pve_pathlen to a non-zero
value on entry, the pathname of the backing object is
returned in the buffer pointed to by pve_path,
provided the entry is backed by a vnode. The
pve_pathlen field is updated with the actual length
of the pathname (including the terminating null character).
The pve_offset field is the offset within the backing
object at which the range starts. The range is located in
the VM space at pve_start and extends up to
pve_end (inclusive).

The data
argument is ignored.

ARM MACHINE-SPECIFIC
REQUESTS
PT_GETVFPREGS

Return the
thread’s VFP machine state in the buffer pointed to by
addr.

The data
argument is ignored.

PT_SETVFPREGS

Set the
thread’s VFP machine state from the buffer pointed to
by addr.

The data
argument is ignored.

x86 MACHINE-SPECIFIC
REQUESTS
PT_GETXMMREGS

Copy the XMM
FPU state into the buffer pointed to by the argument
addr. The buffer has the same layout as the 32-bit
save buffer for the machine instruction FXSAVE.

This request is
only valid for i386 programs, both on native 32-bit systems
and on amd64 kernels. For 64-bit amd64 programs, the XMM
state is reported as part of the FPU state returned by the
PT_GETFPREGS request.

The data
argument is ignored.

PT_SETXMMREGS

Load the XMM
FPU state for the thread from the buffer pointed to by the
argument addr. The buffer has the same layout as the
32-bit load buffer for the machine instruction FXRSTOR.

As with
PT_GETXMMREGS, this request is only valid for i386
programs.

The data
argument is ignored.

PT_GETXSTATE_INFO

Report which
XSAVE FPU extensions are supported by the CPU and allowed in
userspace programs. The addr argument must point to a
variable of type struct ptrace_xstate_info, which
contains the information on the request return. struct
ptrace_xstate_info is defined as follows:

struct
ptrace_xstate_info {

uint64_t

xsave_mask;

uint32_t

xsave_len;

};
The xsave_mask field is a bitmask of the currently enabled
extensions. The meaning of the bits is defined in the Intel
and AMD processor documentation. The xsave_len field reports
the length of the XSAVE area for storing the hardware state
for currently enabled extensions in the format defined by
the x86 XSAVE machine instruction.

The data
argument value must be equal to the size of the struct
ptrace_xstate_info.

PT_GETXSTATE

Return the
content of the XSAVE area for the thread. The addr
argument points to the buffer where the content is copied,
and the data argument specifies the size of the
buffer. The kernel copies out as much content as allowed by
the buffer size. The buffer layout is specified by the
layout of the save area for the XSAVE machine
instruction.

PT_SETXSTATE

Load the XSAVE
state for the thread from the buffer specified by the
addr pointer. The buffer size is passed in the
data argument. The buffer must be at least as large
as the struct savefpu (defined in x86/fpu.h)
to allow the complete x87 FPU and XMM state load. It must
not be larger than the XSAVE state length, as reported by
the xsave_len field from the struct
ptrace_xstate_info of the PT_GETXSTATE_INFO request.
Layout of the buffer is identical to the layout of the load
area for the XRSTOR machine instruction.

PT_GETFSBASE

Return the
value of the base used when doing segmented memory
addressing using the %fs segment register. The addr
argument points to an unsigned long variable where
the base value is stored.

The data
argument is ignored.

PT_GETGSBASE

Like the
PT_GETFSBASE request, but returns the base for the %gs
segment register.

PT_SETFSBASE

Set the base
for the %fs segment register to the value pointed to by the
addr argument. addr must point to the
unsigned long variable containing the new base.

The data
argument is ignored.

PT_SETGSBASE

Like the
PT_SETFSBASE request, but sets the base for the %gs segment
register.

PowerPC MACHINE-SPECIFIC
REQUESTS
PT_GETVRREGS

Return the
thread’s ALTIVEC machine state in the buffer pointed
to by addr.

The data
argument is ignored.

PT_SETVRREGS

Set the
thread’s ALTIVEC machine state from the buffer pointed
to by addr.

The data
argument is ignored.

PT_GETVSRREGS

Return
doubleword 1 of the thread’s VSX registers VSR0-VSR31
in the buffer pointed to by addr.

The data
argument is ignored.

PT_SETVSRREGS

Set doubleword
1 of the thread’s VSX registers VSR0-VSR31 from the
buffer pointed to by addr.

The data
argument is ignored.

Additionally,
other machine-specific requests can exist.

RETURN VALUES

Most requests return 0 on
success and −1 on error. Some requests can cause
ptrace() to return −1 as a non-error value,
among them are PT_READ_I and PT_READ_D, which return the
value read from the process memory on success. To
disambiguate, errno can be set to 0 before the call
and checked afterwards.

The current
ptrace() implementation always sets errno to 0
before calling into the kernel, both for historic reasons
and for consistency with other operating systems. It is
recommended to assign zero to errno explicitly for
forward compatibility.

ERRORS

The ptrace() system call
may fail if:

[ESRCH]
•

No process having the specified
process ID exists.

[EINVAL]
•

A process attempted to use
PT_ATTACH on itself.

•

The request argument was
not one of the legal requests.

•

The signal number (in
data) to PT_CONTINUE was neither 0 nor a legal signal
number.

•

PT_GETREGS, PT_SETREGS,
PT_GETFPREGS, PT_SETFPREGS, PT_GETDBREGS, or PT_SETDBREGS
was attempted on a process with no valid register set. (This
is normally true only of system processes.)

•

PT_VM_ENTRY was given an
invalid value for pve_entry. This can also be caused
by changes to the VM map of the process.

•

The size (in data)
provided to PT_LWPINFO was less than or equal to zero, or
larger than the ptrace_lwpinfo structure known to the
kernel.

•

The size (in data)
provided to the x86-specific PT_GETXSTATE_INFO request was
not equal to the size of the struct
ptrace_xstate_info.

•

The size (in data)
provided to the x86-specific PT_SETXSTATE request was less
than the size of the x87 plus the XMM save area.

•

The size (in data)
provided to the x86-specific PT_SETXSTATE request was larger
than returned in the xsave_len member of the struct
ptrace_xstate_info from the PT_GETXSTATE_INFO
request.

•

The base value, provided to the
amd64-specific requests PT_SETFSBASE or PT_SETGSBASE,
pointed outside of the valid user address space. This error
will not occur in 32-bit programs.

[EBUSY]
•

PT_ATTACH was attempted on a
process that was already being traced.

•

A request attempted to
manipulate a process that was being traced by some process
other than the one making the request.

•

A request (other than
PT_ATTACH) specified a process that was not stopped.

[EPERM]
•

A request (other than
PT_ATTACH) attempted to manipulate a process that was not
being traced at all.

•

An attempt was made to use
PT_ATTACH on a process in violation of the requirements
listed under PT_ATTACH above.

[ENOENT]
•

PT_VM_ENTRY previously returned
the last entry of the memory map. No more entries exist.

[ENAMETOOLONG]
•

PT_VM_ENTRY cannot return the
pathname of the backing object because the buffer is not big
enough. pve_pathlen holds the minimum buffer size
required on return.